Complex electrical/electronic sensors or devices for merely measuring the tension to which a yarn is subjected while being fed to a textile machine are known. Devices are also known which, on the basis of a set yarn tension and the measured yarn tension, control actuators acting on the yarn, to maintain said set tension constant with time independently of yarn velocity variations requested by the textile machine (low or high velocity) or of the variation in the tension of the yarn unwinding from a usual bobbin (between bobbin full and bobbin empty). Said devices, usually known as constant tension yarn feed devices, are hence able to feed a yarn to a textile machine with constant tension independently of any external factor, such as yarn velocity, yarn type, yarn unwinding irregularity, irregularity in yarn take-up by the machine, etc. These devices consequently also maintain this tension constant independently of the manner in which the textile machine is operated, the machine itself operating in accordance with a preset procedure, which is independent of the yarn tension control and regulation procedure.
In such textile machines and in particular in a knitting machine (for example a multi-feeder machine) it is also known to regulate the stitch length, usually by adjusting a simple screw acting on a cam (or equivalent mechanical member) operating with members such as needles or sinkers, in order to adjust the stroke of these to determine the length of the loop or stitch. In other words, the stitch length can be modified by regulating the spatial position of these stitch adjustment members. This adjustment is necessary because an error in stitch length adjustment even on only one of the feeders of the textile machine is known to cause a visual defect in the uniformity of the knitting produced. Moreover on article producing machines, such as hosiery or knitting machines with separation, a wrong length adjustment causes a variation in the size of the article produced, and hence a stocking or vest which may be longer or shorter, or wider or narrower.
A stitch length variation subsequent to its precise setting can be due to many reasons, for example a simple change in the temperature of the environment or of the textile machine itself, which starting from cold becomes increasingly hotter during operation, causes inevitable expansion or deformation of the materials used in the machine construction, and hence a more or less evident variation in the stitch length adjustment. Another cause is related to simple wear of the stitch formation and adjustment members (ie needles, sinkers and cams), which can lead to further variation in an adjusted stitch length. A further cause is related to variation in the tension or lubrication of the yarn fed to the textile machine, which can cause considerable variation in an adjusted stitch length.
Consequently the stitch length has to be carefully and periodically adjusted on all the feeders of all the textile machines present in the production unit, in order to "chase" any yarn take-up variations. However this adjustment is always made either directly or indirectly by a textile machine operator. The adjustment of the screw which acts on the cam operating the stitch members has already been mentioned. This adjustment can be made either by said operator or by a control unit (for example of microprocessor type) which controls the operation of the entire textile machine. In this respect, machines of modern design are known which use actuators of various kinds to adjust the stitch length or to vary it at will according to the production or the aesthetic effect desired. Said actuators are controlled by the control unit, which operates in the following manner. To correctly adjust all the feeders of the textile machine, devices able to measure the velocity with which each yarn is fed to the machine are normally used. In their most simple form, these devices are usually a wheel of known diameter and an r.p.m. counter therefor, to determine the yarn quantity (in meters per minute) absorbed by the machine. This quantity is suitably displayed, and on the basis of this reading the operator can adjust the parameters programmable by said unit, which consequently acts on the stitch adjustment actuators to obtain correct and precise alignment of all feeders.
Hence even in the case of machines controlled by a control unit as indicated, this provides its adjustment action on the stitch members only after the operator himself has set the operating parameters of the unit.
Finally, textile machine yarn feed devices are known which are able to feed the machine with one yarn at constant velocity per feeder. For example, knitting machines already comprise yarn feed devices able to feed the yarn at constant velocity for each feeder.
This is made possible, for example, by a plurality of rotary members (so-called "positive" feed devices) each cooperating with a corresponding yarn fed to a relative feeder. All these rotary members are rotated at the desired speed by a simple smooth or toothed belt driven by a pulley connected by a transmission shaft to the textile machine motor, by which all members are therefore driven. It is hence apparent that having established the correct ration between the textile machine motor and the rotary yarn feed members, when the machine r.p.m. varies a proportional variation in the speed of these members is obtained, to hence give a constant feed ratio.
However because of various problems (already described in relation to the stitch length), this feed ratio does not in reality remain constant with time, with consequent modification of the tension or feed velocity of each yarn to the machine and hence the production of defective articles.
Moreover, measuring a velocity without simultaneously maintaining the tension of the yarn fed to the textile machine constant results in a measurement which has no operational value. In this respect, for example, in an elastic yarn the higher its tension the greater is its elongation and the lower its velocity. Hence while measuring this latter, a variation in the yarn tension can result in an incorrect velocity measurement and hence an unnecessary or mistaken adjustment of the rotational speed of the rotary members (and of the textile machine), or no adjustment at all.